Formation of ceramic mouldings



J1me 1960 M. A. STRIVENS 2,939,199

FORMATION OF CERAMIC MOULDINGS Filed Aug. '7. 1953 3 Sheets-Sheet l F/G./. PARTICLE SIZE-WEIGHT o/sm/eu T/ON FOR M/LLED ALUM/NAS.

WET M/LLED 4 DAYS. MEAN OF3 COUNTS- Z. WET M/LLED DAYS. MEAN OF 3COUNTS.

301?) M/LLED 7DAYS. MEAN 0E3 COUNTS.

v 90 Pg 80- Q 7o- 60%%' m 50- 82 4O- g2 30 Q 20% a /o O I l l l I I I l l l 7 e 9 r0 /2 /3 /4 I5 /6 l 6 MEAN PARTICLE DIAMETER IN M/C'RONS F/GZ.

4x I m rj\ T 7? \1\ /7 T Inventor MA. STRIVENS A ftorney June 7, 1960 M. A. STRIVENS FORMATION OF CERAMIC MOULDINGS 3 Sheets-Sheet 2 Filed Aug. 7. 1953 F/G3D/ST/LLA r/o/v PA 75 FOR MOUL DING FORMUL A T/o/v.2

TEMP. c.

Inventor M. A. STFUVENS Attorney June 7, 1960 M. A. STRIVENS 2,939,199

FORMATION OF CERAMIC MOULDINGS Filed Aug. '7. 1953 3 Sheets-Sheet 3 MIN/n9 Pre-Ca/cin/ngZ M/x/ng 3 Harden [n9 5 Vacuum 6 50/1/e/7L 7 D/stil/ation Extract/0n Inventor MASTRIVENS Attorney FoRMATroN on cnnmvnc MOULDINGS Maurice Andrew Strivens, London, England, assignor to International Standard Electric Corporation, New York, N.Y., acorporation of Delaware Filed Aug. 7, 1953, Ser. No. 372,944

(Ilairns priority, application Great Britain Aug. 11, 1952 19 Claims. (Cl. 25-156) This invention relates to the formation of articles from suitable materials, for instance, ceramic powders, metal carbides, cermets, or mixtures of these including magnet materials, and its object is to'provide a process and suitable formulations of materials for the economical production of articles in large numbers to controlled dimensions.

One feature of the invention comprises a moulding mixture comprising a sinterable material and a vehicle comprising a thermosetting moulding material and a plasticiser so chosen that the thermosetting material can be substantially hardened without driving 01? any significant part of the plasticiser.

The invention comprises a process for the production of articles from sinterable material which process comprises the steps of mixing the sinterable material with a vehicle comprising at least one hardenable component, forming the shape required by the application of heat and pressure, stabilising the shape by hardening the hardenable component, removing the greater part of the vehicle and finally firing the shape.

The invention will be described in relation to various processes and formulations with reference to the accompanying drawings in which;

Fig. 1 shows graphs of particle size v. weight distribution for milled aluminas;

Pig. 2 is a schematic diagram illustrating the injection moulding process;

Fig. 3 is a graph of the distillation-rate for moulding formulation or vehicle No. 2; while Fig. 4 is a simplified flow diagram illustrating the processes described.

The trend towards high quality of ceramics in many fields relies more and more on the use of relatively pure, and hence non-plastic oxide ceramics, so that special methods of manufacture must be employed for such materials. The problem is of course similar with refractory metals.

However, the process is not necessarily limited in its application to nonplastics.

Ease and accuracy of moulding are determined largely by the flow properties of the moulding material: these can be improved by the addition to the sinterable material of suitable moulding vehicle. However, the difficulty then arises of later removing the vehicle Without uncontrolled change of shape or dimension. By means of the types of mixtures and processes to be described, moulding mixtures capable of highly accurate moulding and accurately predictable dimensional changes during manufacture are made available.

The basic technique is to provide for moulding under optimum flow conditions throughout, fixing of the moulded shape without loss of vehicle, removal of vehicle, and final firing, and the vehicle constituents are chosen so as to make this technique possible.

Thermosetting material (including so-called coldsetting material) is included in the vehicle to give the fixing of the shape; plasticiser is included to give easy ted States Patent Patented June 7, 1 96 0 is so chosen that it is not removed during the hardening operation; and a temperature gradient is provided between the removal temperatures of the various vehicle constituents so that they are gradually extracted during the removal operation without risk of collapse of the article.

Hardening without loss of vehicle is dependent on low vapour pressure of the other vehicle constituents, which in turn necessitates a suitable removal process. Such processes are for instance, vacuum distillation, and solvent extraction.

The process is applicable to the production of articles from any ceramic body, though of course the formulation and the process parameters will vary with the body. In particular, it is suitable for semi-conductor, ferromagnetic, ferroelectric, and high-permittivity ceramics.

The main stages in the process are illustrated in the flow-sheet shown in Fig. 4 and may be briefly described as follows:

(N.B.-The numbered stages correspond to the number boxes of the flow-sheet.)

(l) Milling.-Preparation of the ceramic or other sinterable material in suitably sub-divided form.

(2) Pre-calcining.-This does not apply to all sinterable materials, but only to certain mixtures in which a certain number of reactions are preferably carried out in the solid state at moderate temperatures (around 1000 C.) before moulding.

(3) Mixing-The process of mixing the prepared base material with the moulding vehicle.

(4) Moulding.Moulding the mixture to the required shape by any one of well-known techniques, e.g. extrusion, injection, pressing and so on.

(5) Hardening-This is a general term for the process by which the green shape from the moulding, process is rendered permanent, and for a thermosetting component of the vehicle (as defined) comprises a curing process (polymerisation, cross-linking or the like);

At this point, the main process branches into a number of possible difierent processes for removing the moulding vehicle, two of which are to be described:

(6) Vacuum distillation, in which the vehicle is removed at moderate temperature under conditions of low pressure; and

(7) Solvent extraction, in which the vehicle is removed by well-known solvents which may thereafter be extracted for further use.

(8) Firing.-This is the final stage, in which the residual carbonaceous matter, or the bulk of it, is removed by oxidation and the base material (ceramic or the like) is agglomerated by sintering, or densified.

The main example of the production of sintered bodies by the process of the present invention will be that of sintered alumina ceramic, with reference also to a number of other ceramic bodies.

THE PROCESS FOR SINTERED ALUMINA Preparation of the alumina The raw material is fused alumina passing ZOO-mesh per inch, which is wet-ground in a rubber ball-mill of- Sintered alumina balls (50 g. each approx.) kg

4 Sintered alumina balls (20 g. each approx.) kg 3 Fused alumina, ZOO-mesh kg 3 Distilled water litres '1.6

' is 9 ilio f LSSefiOI-Mi.

to a higher or lower shrinkage factor and to work with The milled product is in th e form of a thin cream which i is emptied from the mill into basins and dried at about .0? over i hnp al er ative yri filtere e dld edtz Th (1' -;T .;'he parti size .d o .fq m te e is hewh nF t to e. a p oduc o densi y'a out. 3:5 V

r Prapa ratin o them ouldirig Campanil .ake is. c u hed; betwe r bb sh oller .5

* a dis su ta :for sin in at 78,021809" e is wingformulationsinwhich thefirst:term is sinterable materiaL-which formulations can be moulded other particle size distributionand kinds of sinterable material, or to finish up with a dense or a porous fired body. In these cases the same class of organic materials as described above may be used, due consideration being given to the total volume percentage and relative volume percentages required. e V a The volume ratio of sinterable material to Vehicle may V be as low-as 45/55forevenilowerdn=speoial 'cases,iand

still "yield-a coherent body, and the ratio may be as high as 70/30 for obtaining'den'se bodies and -low shrinkage factors, or even higher consistent obtaining. suitable flow properties.

and take'n to the final firing stage' withoutfdistor 7 914, arenaa at the 291 151 9 e i r a e b T eY P 'f lercent by (.2):,:As. abovezbut with-theWax 365 and butyl stearate replaced by an equivalent weight of glycer'yl mono stearate,: 1.1L; -I

(3) Asrinl (2) but with the ;Araldite resin replaced by BakeIiteiResin'R.. 5363.. v i v 1 i Wax 365 manufactured by Claud Campbell andlCo. Ltdr, although y; u. an flbe ex a be se or 3 m l e q iti n-to be swa at re re,

sents the plasticiseror oneof the plasticisers in the mould: nazyeh e e 1 I Butyl stearate .rnonostearate are obtained from manufacturingchemists. These materials represent plasticisers forming part of the moulding vehicle. :7

The Epikote resin is similar to the Araldit e resin, being also aco'ndensatibn-prc'adddt of epi-chlorhydrin and diphenylolpropane; but, has difierent compatibility characteristicsand curing properties. ,This material is also. a hardenable material 'which'forms a portion of the mouldingvehicleinlixamplefiabovei. l v i The'components of the mixtureare preferably,

thou h; not necessarily, in' a "dnplex mixer, the chamber and: the' blades of are chromeplated and polished? Mixing carriedout at 12i)l3 0 "C; and requires us zominuteno formasatisfacto ry dough. The dough may be granulated by cooling with the machinerunning" and thisftakes abOut'ZOminutes.

iThe above. formulations are 1 instanced merely by wayl of example for .onaceramicpowder. andallinear shrinkage" factor of about 13% However it may be desired to -work -Of the vehicle','-the preferred"volume -percentage 0f hardenable component is generallyabout 2535%. This volume percentage inay" beiiicreased to 50% or more, if desired, but the higher the value is the more diflicult it becomes to. obtain satisfactory flow characteristics, though the dimensional stability-improves.

v.This.,vqlu'me'percentage may, also. be lower than 30%,.

but ,it is, inadvisable to. reduce vthis below; 10% as dimensional stability-will be. very much reduced;

.Ihevolnme ratio of modifier. ,tohardenable component,

7 may, heused tojcontrol the fluidity, of ;the, formulation,

Percent.by wt.-.

- ability.

andjthis factor is especiallyyaluablewhen the 'plasticiser is also compatible with the hardenablecomponent'and.

hemodifier...

Low-ratios are used when .volume percentages of sinterable material or .Whentheparticles exert an,

undue, resistance to flow, K to give maximum -flowability;

the modifier may even be omitted altogether, thuskrelying.

onthe plasticiser,;to control both hardening .and flowl ligh ratios are ns edtwhen using low volume percentages of sinterable material to increase stiifness andv thus prevent flashing inmouldingand to-increase coherence.

In general it is not necessary to increase this ratio beyond about 3 for. a'volume percentageof about; 10% of the na e c po ent... i y

, Injection moulding The moulding Process may be described with reference f to Fig. 2, which shows a sectionthrough part .of a suitable apparatus at a particular stage of the moulding cycle. 7 r 1 The heat' g cylinder 1 is heatedtoabout 140 C. by means oiaband heaterS and thermostatically controlled by means of an element 6. The injection plunger2 with a false tip 351's shown in thejforwa'rd position, the mould cavity 1 2 having just. beenfilledi The plungeris. main: tainedinthis position under pressure for a definite period known as the plunger forward time. ,Thisperiod will dependon thesi ze 'and shape of themoul d cavity, ,but is generally of short duration, about one second. Atthe end of tbegplungerforwardtime; the-cylinder and plunger retract to the normal position, .withjthe plunger tip at the rear ofthe feedchannel ,Thisallowsthe next chairgepfrmaterialfito fall'in frontof the plunger. The

ratcpf. feed issoadjusted-that the cylinder always contains at least 3 to 4shots of material.-, 7 V

-.Th item hal fi e ma ithe ree-p t s bolted to. afixed platen of the, press, while the rear-hal f the bolster 10, isbolted to a r noving platen. From the beginning of the plunger forwardp'eriod, the cooling period forithe moulded part com ences. also, depends on the size and "shap'e of 'the icavityg and may vary from about 2 seconds up to 15 seconds. Attheend of the cooling period t'he'mouldiis opened. the mould opens the Z-pin :16 pulls the material fromthe sprue channel 15, thus clearing the-latter for the-next cycle. As the bolster approaches its rearward position, the ejector pin 17 moves the ejectori llland Z-pin forward, ejecting the moulded part -together with therunner 13) and gate 14.

These are removed by hand and the mould'is closed for thenext 1 cycle.'-- The space immediately in front of. the

Z-pin serves as a Well to collect the first material, which may be slightly chilled coming from the tip-of the nozzle, thus eliminating the possibility of blocking the gate.

The moulded parts have a high green strength and may be handled and stacked, ready for the curing stage, without difiiculty.

Curing The moulded parts are suitably supportedin trays or boxes and heatedto cure the thermosetting constituent of the moulding compound. The parts may be placed in an oven at substantially the moulding temperature and the temperature raised at about 15 C. per hour to a peak curing temperature and held there for a period sufficient to cure its thermosetting constituent. The peak temperature should not exceed about 220 C.

The method of supporting the parts during the curing stage depends on the shape. The flow properties of the material when hot and at the beginning of the curing stage are such that the material will not flow unless subjected to some force. If the moulded part is of sufiiciently thick section and has a large fiat surface for support, it may be cured by supporting it on this surface, without distorting under its own weight.

In general it is preferred to embed the parts in granular fused aulmina. This ensures that all surfaces are adequately supported whatever the shape. There is practically no shrinkage during the curing stage (about 0.1%) so that parts having holes or recesses are in no danger of being cracked. There is a small loss of volatile matter during curing and embedding ensures a uniform loss per unit area over all surfaces.

Low pressure distillation or solvent extraction The greater part of the moulding vehicle is removed from the moulded parts by low pressure distillation or by solvent extraction. The partsvmay be satisfactorily fired when only 50% by weight of the moulding vehicle has, beenv removed. It is preferred however to remove about 90-95% and recover the maximum material for re-use. The remaining fraction consists of a carbonaceous residue which is burnt off in vthe early part of the final firing.

For low pressure distillation, the parts may be stacked, as for the curing stage, or embedded in granular alumina, as again there is practically no shrinkage. They are placed in a vacuum chamber provided with internal heaters and thermocouples. The chamber is pumped down to a pressure of about 2 to 5 mm. Hg and the temperature increased gradually according to a predetermined schedule. It is not possible to give a general rule for the temperature-time schedule, since the temperature distribution through the charge must be known first, and this will depend on the design of the heating arrangements and the size of the charge, and also on the shape and section of the moulded parts. The total distillation time has been as short as 8 hours for small parts and as long as 20 hours for larger parts. The critical range of temperature is between 200 and 250 C., since in this range both the Araldite type and the Bakelite type of resin tend to foam and the overall rate of distillation is high. A temperature rise of about 10 C. per hour is considered to be safe in this range, provided excessive temperature gradients are not allowed to develop in the moulding.

The distillation rate for moulding formulation vehicle 2 was determined and is shown in Fig. 3. The points on the curve represent thecumulative loss in weight after heating for 1 hour at the specified temperatures and at a pressure between 2 and 4 mm. Hg.

The solvent extraction process is one of possible alternatives to vacuum distillation, and has certain advantages over that process.

.In the extraction process, the formed parts are subjectcd, with or without prior heat treatment, to extraction by means of a suitable solvent or solvent vapour to remove the major part of the moulding medium, and finally fired by ordinary ceramic or powder metallurgical methods.

At least two components must be comprised in the moulding vehicle, at least one of them being insoluble or only slightly soluble in the extraction solvent at the time of extraction, and the remainder being soluble.

Organic moulding media as described in the above formulations may be used.

Suitable solvents are acetone, amyl acetone, methyl ethyl ketone, carbon tetrachloride, trichloroethylene.

The extraction may be carried out by placing the parts in a cage and immersing them in the boiling solvent, or running hot solvent or solvent vapour over the parts stacked in a suitable container. After extraction the parts may be simply air dried or oven dried for a short period (15 minutes) to remove residual solvent, before firing.

Extraction may also be carried out by placing them on a suitable conveyor and passing the conveyor through a zone of solvent vapour or under a drip feed of hot solvent and thence through a drying zone.

Although the theory of the process is not known with exactitude, it is believed that the insoluble part of the moulding vehicle forms an independent matrix, which by proper selection and proportioning of the components of the moulding vehicle, is strong enough to give the parts adequate coherence and mechanical strength to prevent deformation cracks and blistering in processing etcror breakage due to handling. The volume percentage of this matrix is small enough for the matrix to be burned off without causing deformation etc.

The advantages of the process of solvent extraction may be summarised thus:

(a) The removal of the moulding vehicle is faster and requires less expensive andrmore easily maintained plant.

(b) The major part of the moulding vehicle is recovered in the same form as that in which it enters the moulding compound.

(0) Defects in treated parts are more easily detected, facilitating rejections before they enter the firing stage.

(d) Where machining of parts before firing is required, there is'less tendency to clogging of the tools using solvent extracted parts than with parts which have only been heat-treated to harden them.

(2) Relatively high mechanical strength of extracted parts.

Firing After removal from the previous stage (vacuum distillation, solvent extraction or the like), the parts still have a high green strength, though it is sometimes less than that of the cured parts, depending on the period of treatment.

The method of stacking the parts for the firing stage determines more than any other step the limits of dimensional accuracy. Sintered alumina above 1000 C. begins to show some ductility and at 1900 C. behaves like a perfect plastic body when under stress, a fact reported upon by E. Ryshkewitch in the Journal of the American Ceramic Society, vol. 34, No. 10, of 1951, at p. 235. An examination'of commercial sintered alumina ware often shows that it has sagged due to plastic flow and insuflicient support. It is important therefore in order to maintain dimensional tolerances to provide adequate support and allow for the fact that during the shrinking the parts Will move relative to the support.

A good method of support is to embed the parts in fused granular alumina of a suitable. grain size, but this method cannot be used Where the part has holes or recesses, since it will shrink round the grains and, crack itself. 7

Flat plates containing holes have been successfully fired by stacking them in a pile inside a saggar (a lidded container e of refractory material) having previously 3 dusted the surfaces with avery small quantity of zircon flourlto' minimisesticking. The-zirconreact's with the aluminmiorming a complex silicate having no known deleterious effects but allows'the parts to be separated by gently tapping the edges'with-a hard object.

* Some shapes may be stackedin the ordinary way inside -a saggar, where-the section of the part is thick enough to support its own'weight, without distortion.

Small terminal insulators (about AM overall) have been fired by simply l heaping them in' an alurnina boat.

The furnace u'sed is preferably a gasfired batch kiln,- the 'chamber being avertical "cylinder with-an air-gas mixture entering tangentially ata thebottom; 1 Provision for anoxyg'en supply is a-useful addition. -Any-other: suitableturnace may, howeven' be used, but furnace atmosphere plays important part in the results achieved-. a

The firing schedule for sintered alumina is fairly short.

After'a'n 'initialburningofi period of about l hourat-a temperatu'rbetween'400 C- and 600 Czthe temperature may'be'raised to'1200 C. in about 2 hours. From 1200 C. to the peak l780-1800 C.,: the rateof 'tem perature-rise should-not exceed 200 CQper hour as all the shrinkage takes place in this range. The peak temperature is maintained-for about b hour to l hour and the furnace-may thenbe allowed tocoolovernighhgor. may be force cooledin a few hours by cutting off the gas supply-and reducing theair -fiow to a= suitable value. Forced cooling, provided it is nottoo'rapid; has nodeleterious eliect either on the ware-orthe furnace, provided the furnace refractories used have been-carefully chosen. In one such"furnace, the--chamber was builtfrom interlockingcurved: bricks made of sillimanite with a zircon inner facing, and this furnace gave good service.

lf'he ceramieprocess, substantially as described above,

has been applied' to' a number-of other ceramic bodies in addition to pure alumina,:and here it is desirable to explainfurth er featuresofthe processof more general 40 always possible to select the distribution" required to pro duce'the desired results because the materialbecomes very difficult to methods. 1 a

of fabrication into the desired shapeb'ecomes far less dependent upon'theparticle size distribution andupon' the natureof the particles themselves. For'example one does not have to rely on the natural plasticity of clays and similar materials-to give the desired flow'properties" for fabrication. Again many of the solid state reactions 1 may be'brought to an'advanced stage prior to fabrica: tion of the -shape,' -by' calcination," thus. eliminatinga number of variable factors *afiecting the shrinkage" of the moulding e.g. changes-from one crystalline ,form'to a and carbon dioxide.

denser crystalline form, andfloss of'coi'nbined moisture V i Preparation the ceramic body: 'S ome ex'amples of types of cerarnic body which have been treated by ceramic injection moulding by the ceramic process'described areas follows: i

' :Purezalumihabodyqi I (2)1;High alumina. ffluxed body.

fabricate by the traditional ceramic By the ceramic process described, however, the case -(4) Rutile-bodyp (5')- Steatite.,body.- i U (6)'Alkaline Earth body. I -(7-) Wollastonite body. z

Pure alumina body h sc c urse, th subj c of th 5999ifi QmbOd tz t si c yt dcs iblcdt bove butfiillawbc di s cdiu. mo d il a a, raw mat ia n h qfit i process. M Q

:Fo t i Q vi b er ab e t6 ram. tame;

fi eal mi a, a hcu 'the rai s zcscl tcd ll-de en pont c pplicati n n vi t-v Fo .c amp crwh rcincnr porosity is not essential, a coarser; startingmaterialand a coars rm led. m ter a m y he u d; butte a h h s mat rial, as pr d yth abcvc.p .occss. et

starting material, e.g. that passing about ZQO mesh per inch, de i e an .a.m an cc sthat,reduces,tQ

ah artic s z ma n y l s ,t an-t ..mic cn -.(-1-.,

micron;1 0 metre;0.00004 in ch).

. Th maximumparticle size which Will duction of non absorbent and highly densepurealumina ware varies somewhat according to the authorities studied. These authoritiesvary, in their criteria of particle size which can be tolerated, and thefdlstribution of particle sizes-below the, ma imum, qsrwell. as. in thel. peakfiring temperature. .Figuresrangeiromga maxi-.. mum diameter of 40. microns with. the, majority less than 10 microns and. a firing temperature of. 1850.";v C. to-

particles predominantly in the ,range,,0.5. .to. .3 microns,- and a firing temperature between 1700" C. and 1800" C. In the process" herein described, the 1 alumina used, wet

milled l daystsee Fig. 1) has less thanv 3,microns (size-weight curve). However is evident bothtromlthe literature andontheoretical. grounds that increasing fine I mess reduces the siritering temperature necessary to, pro; duce the maximum densityin the fired ware, ,ithough according to Ryschkewitch, mentioned above, it doubt P aatb br li tb p 177,

ful ,Whetherthis temper C. for pure alumina;

Ultra-fineness, howeve fis not to be pursued regard T less .of other considerations, since 1 the resulting high specific surface area for: a given concentration in ,the; moulding vehicle may confer an unduly high viscosity on the'latten making it difli cult to inthe moulding opera: tion. This was the case with the rutile body (q. v.)

Ultra-fineness may also result in an'undesirable .gr inte iture the fired body, aresult reported onby Duvez, Odell and Taylor in]. Amer. Cer. Soc." 32 (1949),.

, Most crate work on pure sintered alumina reported;

inthe literature has involvedthe use o f steel mills and balls asthe grinding media for reducing the particle, size. This entails considerable contamination by; iron.: However since much of {the processing; is by means by. slip-castingqusing. hydrochloric" acid. slips, the contamination' may be. conveniently removed, pri0r=to-casting, by

washing with hydrochloric acid and adjusting the pH. For injection moulding of'alumina, the 'latter is com bined with organic materials and it is more convenient to use a milling procedure which avoidsmetal lic com tamination." This'was achieved in the present invention by using a rubber ball-mill and 'sinteredaluiriiria'balls asthe grinding media. Both dry-milling and wet-milling were tried for various milling periods'and some' typical.

particle size-weight distributions obtain ed areshown in Figure: 1. In these experiments all m lling conditions.

were constant except of course for." the lmilling time and the omission ofwater in the. dry milling. ,-From the results it is evident that'wet-milling. hassa distinct advantage over dry-milling in that a muchv larger .porpor-.

tionoffinesis obtained over a shorter period. i However,

wetfmilling .for. prolonged .periods gives only a small in- Q crease in the weight percentage of fines and may be uneconomic on account of the time factor.

The density of about 3.5 g./ cc. for pure alumina ware is not the optimum for the desirable physical properties of strength, non-porosity and surface smoothness, but is very well suited for some applications, e.g. valve electrode spacers Where ease of degassing, high thermal shock resistance and high resistance to surface cracking are required. Pure alumina from which the majority of particles above microns had been removed by sedimentation moulded normally and gave a product having a fired density of 3.74 g./ cc. for a firing temperature of 1800 C., which is suitable for gas-tight, smooth and strong alumina ware.

The four-day wet-milling period previously referred to was selected as providing a suitable compromise among the various requirements of milling efiiciency, mouldability and the sintering properties. A general all-round increase in eificiency, combined with some degree of control of the upper limit of particle size at any desired value, can be achieved by the use of a continuous-flow wetmilling plant in combination with settling tanks.

Alternatively, a separate sedimentation stage after the wet-milling may be introduced as a batch process, whereby an approximately 70% by weight theoretical yield of material less than 10 microns in particle size may be achieved.

High alumina fluxed body The high alumina fiuxed body uses the Wet-milled alumina with the addition of a suitable ceramic flux. The follovw'ng batch composition is given by way of Example 1:

Percent w.w.

Alumina (wet-milled 4 days) 95 Calcium oxide 3 Bentonite 2 This type of body, consisting of a mixture of clay, kaolin, flint and feldspar, contains a good deal of colloidal matter and moisture in the initial mixed state and is calcined at 900 C. on that account and remilled before incorporating in the moulding mixture. Flint pebbles are used as the milling medium.

The material moulds well and fires satisfactorily at 1200 C.

Rutile body Anatase was used as the starting material for this type of body, and this was calcined at 1000 C. to convert it to the rutile structure and increase the particle size and then remilled, using alumina balls or flint pebbles.

The material moulded very well and could be fired satisfactorily to a dense body.

Steatize body This body is based primarily on a high quality talc with minor additions of fluxes. A typical formulation which was selected on the basis of a very low dielectric loss angle consisted of the following batch composition:

Parts by weight Talc 60 Kaolin Magnesium carbonate 7.5 Barium carbonate 17.5

The components were mixed by wet-milling, using steatite balls, and calcined at 950 C; for l /zhours.

The calcined body was remilled before incorporation-inthe moulding mixture. This formulation illustrates'particularly well how the flow properties and'theshrinkage factor may be controlled by selection of the particle size distribution and the filter content. In this case it has been found that for a' given flowability or mouldability, determined by means of a standard mould and moulding conditions, the filter content of the moulding compound and hence the shrinkage. factor is a smooth function of the milling time of the calcined batch composition. That is for milling times within the range of 1 hour to 120- hours, the corresponding filter contents increase from about 62% by wt. to about by wt. for equivalent mouldability. The shrinkage factors decrease correspondingly from about 25% to about 8%.

Alkaline earth bod This body is based on kaolin, flint and alkaline earths. A typical formulation which was selected on the basis of defined surface properties consistedof the following formulation:

Parts by weight Kaolin 71.0

Flint 13.0 Calcium carbonate 4.0 Strontium carbonate 4.0 Magnesium carbonate 4.0 Barium carbonate -Q 4.0

The conditions of preparation depend very much on the nature of the ceramic surface required, which pri-' marily is determined by the precalcination conditions. A precalcination temperature of about 1150 C. yields a ceramic surface eminently suitable for preparation of cracked carbon resistors. The filter content of the moulding compound is then derived as a function of milling time as in the previous example, and the milling time chosen to give the required shrinkage factor. Flint pebbles are used as the milling medium.

Wollastonite body This body is based on the mineral calcium metasilicat'e and a suitable batch formulation is as follows:

Parts by wt. Wollastonite 70 Kaolin 2O Barium carbonate 10 The main requirements of the moulding vehicle are as follows:

(1) Theme-hardening with a sufficient thermoplastic life to maintain the initial mobility for a minimum period of about 1 hour;

(2) Sufiicient mechanical strength combined with low creep to allow for removal of the part from the mould without distortion or breakage;

(3) Low viscosity in the softened state;

(4) Suitable distillation properties;

(5) Low adhesion to metal parts;

(6) Cheapness.

The. thermoplastic life of 1 hour, as defined above, is necessary to allow for the mixing operation, which takes about three-quarters of an' hour, and for the mould- @3111, its 01-5 s-A --Ih r .iare twotimainitypes of it ermes in vre mi; those-which .p ym rise u d th act fhe alone; andthosewhi h r quires'ithe addition. o a i r ss in i ent and/0.1 asatalyscnith brrwit ou t e ppl cat n; of h at-i. Among the kn w t erm e ng resin ere; reinone, whichcoul s ti fy a l heabove r quirem ts. imultaneously; Gen ra ly 1. an.d, :2 ca ief met a '11 ingz p afion', sw thout: deterioratien. .of the. flew pro 1 taneouslyibut, t ese are n t comp ibl w t 3 A n .5

fora numb'enofrea onst: ir tlyt eflw pr pe t s very sensitive to changes in the ,degreeof polymerisation in'litsiearly; stag aseco ytm r s ns-a e ge a l e er a sticky, a d la y, I s a ngle-ph e t ey ld have.

poor distillati n pr pertie de ompos n d dis la ion taking place over'a, narrow temperature range, resulting in disruption of the ware However, by diluting thetheimosetting resin with a thermoplastic resin the. concentration of polymerisable nuclei rn'ayibe :controlled andjthe flowpropertiesmade less; sensitive to; change in .the' .degree of polymerisation In addition, by using a thermoplastic 1 in its early stages. resin ,of lower. softening range than that of the thermosetting. resin, the working range of temperature for, mix:

ing/andmouldingtmay be increased ,andthe distillation characteristics improved. ,The, user of; ,a thermosetting; resin and. aithermoplastic resiniinicombination therefore gives a vfair degreeuof control, over the propeitiesof themoulding .vehicleand a wide, choice ofMtheotherficom-j porlents. a

iReqiii'rernent's sand 5 are not met sufficiently well in practic'iwithout 'theuse'of a third typeof component 1 foi' tli'euno'ul'ding vehicle which may 'be"called a plasti This consists of a1wax'Ior oil or mixture thereof, j 'ts' as' a blending agent for'theitwo resins, reduces the ,v' sco'sityi by jvirt'ue' of its low "melting 'point, gives good 'rel aseprepen es fromf metal'i surfa'ces, and

provesthe distillation characteristics;

The preferred basic"-formulation [for the moulding vehicle may be summarised as follows: o (1) A thermosetting resin,-=with setting range T T an me t e eTa.s. 4

(2) A thermoplastic'resin, with rnelting range;T T

(3) A plasticiser with melting range T -T where and Where T2T3 is V suificientlylarge to 'give a thermoplastic life of about 1 hour atthe concentration'chosen: i

*Itisthe properties of-thethermosetting resin which determine the selectionj of the other components,j and the optimumpropertiesof the'monlding formulation are,

developed. frQm-;this staget by a study of therelative.

mutual compatibilities -rof the; components and ,by yariag tion- .of the relative concentrations ,to produce the best compromise tosuit the requirements ,listedabove, ,Referring ;to .thepreferred basic formulation above,

the three basic components :need not. necessarily be IfiP': resented only, by a ,single' member of their type. 1 The Epikote resin. Also the plasticiser may comprise ascatalyst for the condensationof two resins, or the polymerisation of the thermosetting resin; a, '1 f .1

Curing and loiv pressure distilln tign Ceramic :injectioni moulding processes are' known i h. use,,-;bak n t mes :(a d s in t rs fifi t me ary ng fromabout itill ut i 9 9. .;l? i 19? nown whe herthey s l w pre sur .d ti lat qntthqqshs itisunlikely. r 7

aIh d v si nr08 ;v be s ean ,1 :lqw:M ssrs-d st la ns a eA th rrvs s cri d results s r siss i tar ,S-,.- d i$2Y Q l s r nss t d cra k n 9i 7 WPQkliB -IW QW I bak 7 the kn wn rrb e sins t ,l i i n s si s a e-sfl. i i ioaa wi r' ui nteasi ti ps the ki u rbc ssi a c mpl s ed a hqrt rfimfil b t-1 eve tb h a nat era t at: 18 F200? Ci foi l P io ufiifi sa o r the erm t inarje in ndi i re-heating parts at low; pressure slowly frornabout 40. Cite 2 0 l sientex ra fi' i i t Pa t by distillation takes place to preventblistering "Mat o the, rm isisn a w prssisuredistillatidn' stage gives the important; advantagethat a'wi r range of thermosetting resins is made; ayailable' s nce V thermoplastic resins and, plasticisersi"may thenlbe used having, very low vapour fpre'ssure f 3110 m emperatures L L With such .components, i. moulding cnring can be carried out, without significant loss of: plastic material,

and in the low, pressmsarsnnaam stage, removal of the 1 low vapouripress u r e, constituents can befeffected at a relatively 16w tem eraments consequently low vapour 7 pressure, thus keeping the internal gaseous{pressures at 7 allow. value andreducing the risk of distortion or frac ture of the warevery mate'rially. S F Q The results obtained showthatpuresintered alumina ware can be produced t 'o"a' high de ree of dimensional accuracy --and-in a-yery great variety of shapes lay-ceramic injection moulding by the process f described.-

Dimen sional accuracy-has been as great "as "1-0. 002" perinch on some parts, and $0.005" per inch is quite common. C erarn ic injection moulding has the further advantage in ati i ,desiredastasdmd mw li lhi inka e can, be used for yar ipus types of ceramic body by niaking usei of two parameters; (a) thepar t i cl e size distribution and (b) the inorganic content of the moulding compound. For example in the process described 'as appliedt o the production of sintered alumina cerami cg the' volume 'percentage of alumina in the moulding" formulation based on the density of 4 gm./cc; is "approximately60%, giving a shrinkage factor of approximately 13%. Thisfigure is somewhat more than that *using some traditional ceramic processes, but owing to the low viscosity of the moulding formulation at the moulding temperature the uniformity of shrinkage isextremely high and easily controlled; and: owing also to ithe nonwolatile nature of the components, of the moulding formulationgat room" temperature, the volumegpercentage of ceramic and hence the absoluteshrinkage factor are more, easily controlled, The low vvolume percentageand high, sl :lr inkagefactor are in fact an advantage rather thanga disadyan tage-since, variationoin ,particle sizefiistribntion fora given material or for different materials, and hence varia-i;

tion in the density of packingbfthe particles can be allowed for, by increasing or decreasing the volume percentagijso as to yield. a standardfshrinkagejifactor, without increasing the viscosity of the moulding beyo'nd'the limit where fio'w is insufiicientfto give satisfactoryjmoulda ings; This feature enables very close control of dimensional accuracy in manufacture, and the choice of a variety of materials combinedi'withi'low mouldcost's.

Mould wear in thebcavityispractically negligible though carbide or ceramic gating should be used in prod particu a im at tin s theril 51? l a y stoodt .th sides ptio mad ly bywa of? example and, not asa limitation on the .,st;,ope of. the i v ventionhi a 13 What I claim is: 1. A process for the production of articles from sinterable material which process comprises the steps of mixing the sinterable material with a vehicle the components of which comprise a thermosetting resin, a modifier for said resin, and a plasticiser, forming the mixed material and vehicle into the shape required by the application of heat and pressure without appreciable hardening of the thermosetting resin, stabilising the shape by hardening the thermosetting resin Without appreciable loss of vehicle, removing the greater part of the vehicle, and finally firing the shape.

2. A process as claimed in claim 1 wherein the vehicle removing process is a vacuum distillation process.

3. A process as claimed in claim 1 wherein the vehicle removing process is a solvent extraction process.

4. A process as claimed in claim 1 wherein the components of the vehicle have a low vapour pressure under the conditions of stabilising the shape.

5. A process as claimed in claim 1 wherein the hardening of the thermosetting resin is carried out by heating with the application of an external pressure greater than atmosphere pressure.

6. A process as claimed in claim 1, wherein the hardening is efiected by heating.

7. A process as claimed in claim 1, wherein the hardening is efiected by means of an accelerator or catalyst.

8. A process as claimed in claim 1, wherein the modifier is at least partially compatible with the hardenable component in the presence of the plasticiser.

9. A process as claimed in claim 1, wherein the plastiser is a wax or oil or mixture of waxes and oils.

10. A process as claimed in claim 1 wherein the plasticiser has a low vapour pressure under the condition of hardening of the thermosetting resin.

11. A process as claimed in claim 1, wherein the vehicle has a thermoplastic life of at least 1 hour.

12. A process as claimed in claim 1, wherein the components of the vehicle are in the following temperature relationship:

T T T T T T T T wherein T to T is the setting range 'of the thermosetting resin(s) and T to T their melting range; T to T is the melting range of the thermoplastic resin(s); and T to T is the melting range of the plasticiser(s); and wherein T to T is sufficiently large to give a thermoplastic (working) life for the compound of about 1 hour.

13. A process as claimed in claim 1 wherein the said thermoplastic resin reacts as a cross-linking agent with the said thermosetting resin.

14. A process as claimed in claim 1 wherein the ratio by volume of the sinterable material to the vehicle is in the range from 70/30 to /55.

15. A process as claimed in claim 1 wherein the sinterable material is calcined at a temperature below the final firing temperature and ground prior to mixing with the vehicle.

16. A process as claimed in claim 1 wherein the sinterable material comprises ceramic materials in finely divided state and having particles with diameters mainly in the range of 3 microns to 25 microns.

17. A process as claimed in claim 1, wherein the shape is formed by injection moulding.

18. A process as claimed in claim 1, wherein the shape is formed by extrusion.

19. A process as claimed in claim 1, wherein the shape is formed by hot compression or transfer moulding.

References Cited in the file of this patent UNITED STATES PATENTS 2,122,960 Schwartzwalder July 5, 1938 2,363,575 DeLamatter et a1 Nov. 28, 1944 2,434,271 Howatt Jan. 13, 1948 2,446,872 Ehlers Aug. 10, 1948 2,580,708 Wallace Jan. 1, 1952 

1. A PROCESS FOR THE PRODUCTION OF ARTICLES FROM SINTERABLE MATERIAL WHICH PROCESS COMPRISES THE STEPS OF MIXING THE SINTERABLE MATERIAL WITH A VEHICLE THE COMPONENTS OF WHICH COMPRISE A THERMOSETTING RESIN, A MODIFIER FOR SAID RESIN, AND A PLASTICISER, FORMING THE MIXED MATERIAL AND VEHICLE INTO THE SHAPE REQUIRED BY THE APPLICATION OF HEAT AND PRESSURE WITHOUT APPRECIABLE HARDENING OF THE THERMOSETTING RESIN, STABILISING THE SHAPE BY HARDENING THE THERMOSETTING RESIN WITHOUT APPRECIABLE LOSS OF VEHICLE, REMOVING THE GREATER PART OF THE VEHICLE, AND FINALLY FIRING THE SHAPE. 